For me it was encapsulated by doom 3 for the most part. Games with strong dynamic shadows like splinter cell chaos theory and stalker with a mix of shiny specular effects is timeless for me. When I go back to these games the shadows still feel as good as they did when they came out and the lighting perfectly crafting the mood. A lot of modern games now even though their fidelity on paper is much better just always feel extremely flat to me and don't pop out of the screen like they used to. It seems like they're missing contrast and UE5 seems to make this even worse where all the games using it just look like mush
Late 2D art for me. While the early Playstation era has a lot of god-awful looking early 3D games, it's also home to many of the best looking 2D games of all time. Some modern titles can get close to the level of quality those games had but even the best modern pixel art just isn't the same as what the industry was capable of back then because there was an entire development pipeline for 2D art that doesn't exist anymore.